Byssoloma brunneodiscum

The thallus forms a smooth, continuous, dull-green film on living leaves and lacks a distinct marginal hypothallus. Algal cells are chlorococcoid, 5–7.5 μm in diameter. Apothecia are sessile, 0.3–0.5 mm wide, with a plane brown to dark-brown disc that is encircled by a thick, persistent white byssoid margin. Beneath the disc, the excipulum and basal tissue consist of loosely interwoven colourless hyphae studded with crystals that dissolve potassium hydroxide solution (KOH) to produce a positive pink reaction; the hypothecium is pale brown and measures 25–30 μm high. The hymenium reaches 40–50 μm tall; asci are 8-spored and amyloid, while the ellipsoid, colourless ascospores have three transverse septa and measure 11–14 × 3–4 μm.^[1]

Asexual propagation occurs through sessile, wart-like pycnidia (10–13 μm across) that release non-septate, pear-shaped conidia 2–2.5 × 1.5 μm. Thin-layer chromatography detects the chlorinated xanthone compound 2,5,7-trichloro-3-O-methylnorlichexanthone in the thallus.^[1]

The species is known only from montane rainforest on Hainan Island, China, where it grows sparsely on smooth leaf surfaces in the damp, shaded understory at around 960 m elevation.^[1]